Storing rvalue references: should this work?

I'm testing my understanding of lvalue and rvalue references by intentionally trying to break things. So say there is this struct:

struct FooBar
{
    FooBar(int&& number) : rNumber(number)
    {

    }

    int& rNumber;
};

and I create an instance FooBar obj(5). Every attempt to read the reference variable returns the right result (5). The same happens if I use const int& instead of int&&.

I noticed that replacing int with std::string and reading the reference returns an empty string, so I suspect it gives undefined behaviour. Is this so? And if so, why does it work with integers?

Update: I'm creating the instance and reading it like this:

FooBar obj(5);
//FooBar obj("Hello"); // For strings...

std::cout << obj.rNumber << std::endl;

Update 2: It also works if you pass a user-defined type, like this:

struct GooBar
{
public:
    GooBar(int number) : itsNumber(number) 
    {
        std::cout << "In constructor..." << std::endl;
    }

    GooBar(const GooBar& rhs) = delete;
    GooBar(GooBar&& rhs) = delete;

    ~GooBar() 
    {
        std::cout << "In destructor..." << std::endl;
    }

    int itsNumber;
};


struct FooBar
{
    FooBar(GooBar&& number) : rNumber(number)
    {

    }

    GooBar& rNumber;
};

and then creating an instance and reading it like so:

FooBar obj(GooBar(5));

std::cout << obj.rNumber.itsNumber << std::endl;

I think this is interesting, because it gives the following output:

In constructor...
In destructor...
5